A fuel cell is a sort of electric generator that generates electric energy by electrochemically oxidizing fuels such as hydrogen and methanol, and has lately attracted attention as a clean energy source. Fuel cells are classified into a phosphoric acid type, a molten carbonate type, a solid oxide type, a solid polyelectrolyte type or the like according to the kind of the electrolyte to be used. Among these the solid polyelectrolyte type of fuel cell is expected to be widely applied as a power source for electric vehicles or the like because of its low standard operating temperature, as low as 100° C. or below, and its high energy density.
The solid polyelectrolyte type fuel cell is basically composed of an ion exchange membrane and a pair of gas diffusion electrodes bonded to both sides thereof. It generates electricity by supplying hydrogen to one electrode and oxygen to the other electrode while both electrodes are connected to an external load circuit. More specifically, protons and electrons are formed in the hydrogen side electrode. The protons migrate through the ion exchange membrane to the oxygen side electrode, and then react with oxygen to form water, while the electrons flow through a conductor from the hydrogen side electrode and discharge electric energy in the external load circuit. They then arrive at the oxygen side electrode through another conductor, resulting in contributing to the course of the above-described water-forming reaction. Although a required characteristic of the ion exchange membrane is high ion conductivity in the first place, high water content and high water dispersibility in addition to the ion conductivity, are also important required characteristics because protons are considered to be stabilized by hydration of a water molecules when migrating through the ion exchange membrane. In addition, since the ion exchange membrane also plays the role of a barrier to prevent direct reaction of hydrogen and oxygen, low gas permeability is required. Furthermore, properties such as chemical stability to resist a strong oxidation atmosphere during the fuel cell operation, and mechanical strength to meet the requirements for a thin membrane, are also necessary.
Ion exchange fluorocarbon resins are widely employed as a material for the ion exchange membrane used in fuel cells of the solid polyelectrolyte type, because of their high chemical stability. “Nafion” (registered trademark) manufactured by E.I. du Pont de Nemours and Company having a perfluorocarbon as the main chains and sulfonic acid groups at the end of side chains is widely used. Although such an ion exchange fluorocarbon resin has generally balanced properties as a solid polyelectrolyte material, further improvements in the properties thereof have been required with progress in the practical use of the fuel cells.
For example, although higher heat resistance has been increasingly demanded, particularly in motor vehicle applications for preventing catalyst poisoning and improving the cooling effect, it is said that the operation of the present standard ion exchange fluorocarbon resin membrane at 90° C. or above is difficult. Specifically, the above-described higher heat resistance requires improvement of the heat resistance of ion exchange fluorocarbon resin membranes to 100° C. or above, preferably 120° C. or above.
As means to improve the heat resistance of ion exchange fluorocarbon resin membranes, prior art techniques using the addition of reinforcing agents or block copolymerization, such as block copolymerization with PTFE (JP-A-11-329062), the addition of PTFE fibrils (JP-A-60-149631) or inorganic particles (JP-A-6-111827), as well as the formation of SiO2 networks by the sol-gel method (K. T. Adjemianetal, 2000 Fuel Cell Seminar, pp. 164–166), are known. According to these prior art techniques, although heat resistance was improved to some extent by the addition of reinforcing agents in several percent by weight or block copolymerization, the lowering of ionic conductivity in exchange for incrased heat resistance became a problem because of the lowering of an apparent exchange capacity. On the other hand, in addition to the addition of reinforcing agents, a prior art technique wherein cross-linking functional groups are copolymerized with the precursor of an ion exchange fluorocarbon resin membrane (JP-A-7-508779) are also known. According to such a technique, although it is considered that heat resistance can be compatible with ionic conductivity by properly designing cross-linking, there is a problem that cross-linking leads to increase in costs, and in certain cases, the cross-linking reaction takes a long time. As described above, prior art techniques related to the improvement of heat resistance have essential problems, and have not become industrially useful techniques for ion exchange membranes for fuel cells.